Uses custom viewports to generate textures before the first frame is drawn

texture_generator.gd

extends Node2D

var myViewport
var myCanvas

func _ready():
	## Stop main viewport from drawing when we force draw
	## should loop through all other viewports as well
	VisualServer.viewport_set_active(get_viewport().get_viewport_rid(), false)
	# initialize Viewport, needs canvas
	myViewport = VisualServer.viewport_create()
	myCanvas = VisualServer.canvas_create()
	VisualServer.viewport_attach_canvas(myViewport, myCanvas)
	VisualServer.viewport_set_size(myViewport, 64, 64)
	## "active" instructs it to be drawn
	VisualServer.viewport_set_active(myViewport, true)
	
	## create CanvasItem and add to canvas just as in servers tutorial
	var ci_rid = VisualServer.canvas_item_create()
	VisualServer.viewport_set_canvas_transform(myViewport, myCanvas, Transform())
	VisualServer.canvas_item_set_parent(ci_rid, myCanvas)
	var sprite = load("res://icon.png")
	VisualServer.canvas_item_add_texture_rect(ci_rid, Rect2(Vector2(0, 0), sprite.get_size()), sprite)

	## draw it once, this should be bundled into a function
	VisualServer.viewport_set_update_mode(myViewport, VisualServer.VIEWPORT_UPDATE_ONCE)
	VisualServer.viewport_set_vflip(myViewport, true)
	VisualServer.force_draw(false)
	var image = VisualServer.texture_get_data(VisualServer.viewport_get_texture(myViewport))
	var texture = ImageTexture.new()
	texture.create_from_image(image)
	$Sprite2.texture = texture
	
	#exact same as above
	VisualServer.viewport_set_update_mode(myViewport, VisualServer.VIEWPORT_UPDATE_ONCE)
	VisualServer.viewport_set_vflip(myViewport, false)
	VisualServer.force_draw(false)
	var image2 = VisualServer.texture_get_data(VisualServer.viewport_get_texture(myViewport))
	var texture2 = ImageTexture.new()
	texture2.create_from_image(image2)
	$Sprite3.texture = texture2
	
	#prints 0 as no frames have actually drawn
	print(Engine.get_frames_drawn())
	
	#reset the main viewport so everything actually draws to screen
	VisualServer.viewport_set_active(get_viewport().get_viewport_rid(), true)

Ok got it, many thanks.
For now I’ll stick in waiting a bit the draw command complete (as the work is done on a thread, it doesn’t freeze the entire game).
I’ll rewrite the whole thing with compute shaders when Godot 4.0 is out, as it should allow to completely separate textures rendering from the draw process.